This invention relates to a method of and apparatus for the partial oxidation of hydrogen sulphide and to burners for use in the method and apparatus.
So-called acid gas streams containing hydrogen sulphide and carbon dioxide are formed as waste streams in, for example, oil and gas refineries. It is necessary to treat an acid gas stream so as to remove substantially all its content of hydrogen sulphide before it is discharged to the atmosphere. This removal of hydrogen sulphide is conventially performed by the Claus process, in which a part of the hydrogen sulphide content is burned in a furnace to form sulphur dioxide and water vapour; some of the resultant sulphur dioxide reacts in the furnace with residual hydrogen sulphide to form sulphur vapour and water vapour (with the result therefore that some of the hydrogen sulphide is partially oxidised). An effluent gas stream comprising hydrogen sulphide, sulphur dioxide, carbon dioxide, water vapour, and sulphur vapour therefore flows out of the furnace. The sulphur vapour is extracted from the gas mixture by condensation, and the resulting gas mixture substantially free of sulphur vapour is subjected to a plurality of catalytic stages of further reaction between sulphur dioxide and hydrogen sulphide so as to form sulphur vapour. The further sulphur vapour is extracted from the gas mixture downstream of each stage of catalytic reaction. A tail gas containing typically from 2 to 6% of the original sulphur content of the acid gases thereby formed. The tail gas is typically sent for further treatment to remove substantially all the remaining sulphur compounds.
Traditionally, air is employed to support the combustion of the hydrogen sulphide. Typically the air is supplied at a rate sufficient to provide enough oxygen molecules to oxidise completely any ammonia present to nitrogen and water vapour and to oxidise completely any hydrocarbons present to carbon dioxide and water vapour and to effect oxidation to sulphur dioxide and water vapour of about one third of the hydrogen sulphide content of the acid gas. It has more recently been recognised that improvements in the Claus process can be achieved by substituting commercially pure oxygen for some of the air. As a result, the size of the furnace and downstream units can be reduced for a given throughput of hydrogen sulphide.
EP-A-0 486 285 relates to an oxygen-air-hydrogen sulphide burner, for use in the Claus process. The burner comprises a hollow body member having an open distal end and defining a passage through which extends a plurality of first elongate, open-ended, tubular members able to conduct fluid and a plurality of second, elongate, open-ended tubular members also able to conduct fluid, each second tubular member being located within a respective first tubular member. The first tubular members communicate with a source of hydrogen sulphide and the second tubular members with a source of oxygen. The purpose of providing each oxygen tube within a respective hydrogen sulphide tube is to make possible the achievement of particularly good mixing of the oxygen and the fuel and to obtain uniform conditions within the flame. In addition, stable operating conditions can be obtained at relatively low fuel and oxygen velocities.
Although the burner according to EP-A-0 486 285 performs well in practice, we believe that in the Claus process a higher percentage conversion of hydrogen sulphide to sulphur in the gas mixture leaving the sulphur condenser associated with the furnace can be achieved if, in fact, a suitable non-uniform flame is provided. Accordingly it is an aim of the present invention to provide a method of and apparatus for the partial oxidation of hydrogen sulphide in which the burner has a construction which facilitates the attainment of relatively high percentage conversions of hydrogen sulphide to sulphur in the furnace.
According to the present invention there is provided apparatus for the partial oxidation of hydrogen sulphide comprising a furnace and an air-oxygen-hydrogen sulphide burner that fires into the furnace, wherein the burner comprises a main passage for combustion-supporting gas containing air, a multiplicity of spaced apart outer elongate fluid-conducting open-ended tubes extending in parallel with each other along the main passage, each of the outer tubes surrounding at least at the distal end of the burner a respective inner elongate fluid-conducting open-ended tube, the inner tubes extending in parallel with one another, a first inlet to the burner for oxygen or oxygen-enriched air, and at least one second inlet to the burner for feed gas containing hydrogen sulphide, the first inlet communicating with the inner tubes, and the second inlet communicating with the outer tubes, wherein the outlets of the inner and outer tubes are so disposed that, in operation, essentially all mixing of hydrogen sulphide with oxygen and air takes place downstream of the distal end of the burner, and wherein the outlets of the inner and outer tubes are so juxtaposed and dimensioned as to enable there to be maintained in the furnace, in operation, a stable flame with at least one high temperature first stage, and at least one second lower temperature stage, the first stage being more remote than the second stage from a chosen area of the inner wall or walls of the furnace, the chosen area thereby being shielded from the first stage by the second stage, and the said tubes are arranged in two groups, there being a first group of inner and outer tubes which in operation feed the first stage of the flame, and a second group of inner and outer tubes which in operation feed the second stage of the flame, the inner tubes in the internal first group being of greater internal diameter than the inner tubes in the second group.
As a result of its being relatively oxygen-rich, an average flame temperature in the range of 1700 to 2300xc2x0 C. can be achieved in the inner stage of the flame. Such high temperatures are believed to favour cracking, i.e. thermal dissociation, of the hydrogen sulphide into hydrogen and sulphur. As a result, it is believed that a higher proportion of the hydrogen sulphide is converted to sulphur than at lower temperatures. In addition, having a high temperature oxygen-rich first stage also facilitates the complete destruction of any ammonia in the feed gas not only because of the high temperature of the first stage but also because the second stage although relatively oxygen poor can still be operated at temperatures in excess of 1400xc2x0 C. It is highly desirable to effect such complete removal of ammonia since if any of this gas passes from the furnace to any catalytic stages of a Claus process it can form ammonia salts which poison the catalyst or block other units operating at lower temperatures.
Preferably the outlets of the outer tubes in the first group are spaced wider apart than the outlets of the outer tubes in the second group. As a result, proportionately more air tends to be made available to the first group of tubes than to the second group of tubes, thereby facilitating the achievement of the desired oxygen-rich conditions in the first stage of the burner. This result is particularly facilitated if there are more tubes in the second group than the first group; typically there are at least twice as many tubes in the second group than in the first.
Preferably the tubes in the first group may have an internal diameter typically from 1.3 to 3 times the internal diameter of such tubes in the second group. Such an arrangement facilitates in operation the sending of proportionately more oxygen or oxygen-enriched air to the first stage of the flame than to the second stage, thereby particularly helping to achieve a high temperature in the first stage. In such an arrangement, it is convenient that the internal diameter of the tubes in the first group that communicate with the second inlet are of the same internal diameter as the tubes in the second group that communicate with the second inlet.
There are generally two different ways in which the burner can be disposed. In one, the burner fires generally along the longitudinal axis of the furnace. If such an axial disposition of the burner is adopted, the second group of inner and outer tubes typically surrounds all the tubes in the first group. As a result, the flame comprises an inner high temperature core, the first stage, and an outer lower temperature envelope, the second stage. More complex arrangements are possible. For example, the outlets of the tubes can be grouped so that there is one or more intermediate stage between the inner core and the outer envelope. Such an intermediate stage can either be a relatively oxygen-enriched stage, i.e. a second high temperature stage, or, more preferably, an oxygen-poor stage, i.e. a second lower temperature stage.
Alternatively, the burner can have a generally tangential position relatively to the exterior of the furnace. In a tangential position, with a horizontal furnace, there is a greater tendency for thermal damage to be done at the bottom of the furnace than at the top. Such a tendency can be counteracted by having the outlets of the first group of inner and outer tubes positioned generally above the outlets of the second group.
The feed gas is typically a mixture of hydrogen sulphide and carbon dioxide. Water vapour, hydrocarbons and/or ammonia may also be present. It desired, the feed gas may be amine gas from an oil refinery or a mixture of an amine gas with a sour water stripper gas. Amine gas typically comprises at least 70% by volume of ammonia and additionally contains at least 10% of carbon dioxide. Sour water stripper gas is a mixture of hydrogen sulphide, ammonia and water vapour. Other gases may also be present in the sour water stripper gas. If it is desired to treat both gases by the method and in the apparatus according to the invention, they can be pre-mixed. One potential disadvantage of such pre-mixing, however, is that some of the ammonia will tend to flow to the lower temperature stage or stages of the flame and a risk may arise that not all the ammonia will be destroyed. In practice, however, with oxygen operation the lower temperature stage or stages may typically be maintained of a temperature above the minimum for ammonia destruction. In any even, the potential disadvantage can be avoided if the burner is so arranged that there are two second inlets, one being associated exclusively with the first group or tubes, and the other being associated exclusively with the second group of tubes. In this Away, if the ammonia-containing gas (the sour water stripper gas) can all be directed into the high temperature oxygen-rich stage or stages of the flame. Such an arrangement can make it easier to achieve total removal of the ammonia from the feed gas.
Depending on its size, the burner according to the invention may have from 6 to 30 first tubes. Preferably, it has from 8 to 20 first tubes.
Preferably, the inner and outer tubes all terminate in an common plane normal to the axis of the burner. Such an arrangement helps to reduce thermal erosion of the burner. If desired, each of the tubes can be given a tip of heat and corrosion resistant alloy, but this is generally not necessary. Indeed, the burner can continue to operate effectively even with erosion of the tubes.
The main passage of the burner is conveniently defined by a port through which the burner fires into the furnace. Alternatively, the burner may have an outer shell which defines the passageway which is separate from the furnace.
There is generally no need for any special cooling system for the burner. This is because the burner can be readily arranged such that, in operation, flow of the combustion supporting gas containing air is sufficient to provide adequate cooling for the burner.
A burner according to the invention also offers same advantages in mechanical construction as that according to EP-A-0 486 285. Thus, the burner tubes may be fabricated from relatively inexpensive materials, for example, stainless steel. Secondly, fabrication is particularly simple because, there being no end plate, no need to drill oblique orifices arises. Third, the burner is able to cope with sources caused by thermal expansion and contraction since the inner and outer tubular members are typically only secured only at their proximal ends and have three distal ends.
By appropriate distribution of combustion supporting and hydrogen sulphide containing gases between the different stages of the flame, it is possible to ensure that the mole ratio of hydrogen sulphide to sulphur dioxide in the effluent gas from the furnace is in the order of 2 to 1 and therefore able to meet the requirements of a conventional Claus process employing catalytic Claus reaction units as well as the furnace. Further, sufficiently low temperatures can be maintained in the shielding or second stage or stages of the flame so as to avoid any risk of damage to the refractory lining which is typically employed to protect the inner walls of the furnace without compromising ammonia destruction if that gas is present. Moreover, there is considerably flexibility in selecting the stoichiometry of the different stages of the flame. Indeed, it is generally preferable to operate the first stage or stages such that they receive oxygen molecules at a rate in the range of (110x/300+y+z) to (240x/300+y+z)m3sxe2x88x921 where:
x is the stoichiometric flow rate of oxygen molecules required for complete oxidation of the hydrogen sulphide entering the first stage:
y is the stoichiometric flow rate of oxygen molecules required for complete oxidation of any ammonia entering the first stage:
z is the stoichiometric flow rate of oxygen molecules required for complete oxidation of any hydrocarbons entering the first stage above.
The temperature of the effluent gas leaving the outlet of the furnace can readily be kept below 1650xc2x0 C.
Another advantage of the method according to the invention is that it has the ability to cater for quite widely varying flows of the feed gas containing hydrogen sulphide. At lower rates of supply of the feed gas, the mole fraction in the oxygen or oxygen-enriched air which is supplied to the first stage or stages of the flame can be reduced. Indeed, at the lowest rates of supply of the feed gas, air can be substituted for the oxygen or oxygen-enriched air supplied to the first stage or stages of the flame. Provided that such a substitution does not prejudice the complete destruction of ammonia, particularly in the second stage or stages. Examples of burners which enable all the ammonia to be fed to the first stage or stages are therefore preferred when good turndown characteristics are required.
The invention also provides a burner for use in the method and apparatus according to the invention.